Propofol (2,6-diisopropylphenol or 2,6-bis(1-methylethyl)-phenol)) is an injectable potent, short-acting, non-barbiturate sedative-hypnotic agent for use in the induction and maintenance of anesthesia or sedation. Intravenous injection of a therapeutic dose of propofol rapidly induces anesthesia usually within 40 seconds from the start of injection. As with other rapidly acting intravenous anesthetic agents, the half-time of the blood-brain equilibrium is approximately 1 to 3 minutes and this accounts for the rapid induction of anesthesia.
Propofol can undergo an oxidative process to produce the dimer, 4,4′-dihydroxy-3,3′,5,5′-tetraisopropyl-biphenyl. This dimer is visually detected at low concentrations by the appearance of a yellow color in a solution containing propofol. The degradation of propofol is thought to occur through a free radical reaction that may be catalyzed by light, oxidation or the presence of divalent or trivalent cations.
The currently marketed DIPRIVAN® formulation of propofol is an opaque oil-in-water emulsion containing lipids and egg lecithin as an emulsifying agent. The DIPRIVAN® emulsion-type formulation contains 10 mg/mL propofol, 100 mg/mL soybean oil, 22.5 mg/mL glycerol, 12 mg/mL egg lecithin and 0.005% w/v disodium edetate (EDTA) at a pH of 7.0 to 8.5. It is packaged under nitrogen headspace in single use containers. Even though it is a sterile product, the DIPRIVAN® formulation can support microbial growth as it is not an antimicrobially preserved product under USP standards. The formulation also has problems concerning allergic response to the egg components, and the moderate to high incidence of pain on injection.
Emulsion formulations are typically problematic with regard to microbial growth, not only because the lipid components can readily support the growth, but because 0.22 micron or smaller “sterilizing” filters cannot be used. A filter pore size of ≧5 μm is recommended for the marketed formulation “unless it has been demonstrated that the filter does not restrict the flow . . . and/or cause the breakdown of the emulsion” (DIPRIVAN® Injectable Emulsion Propofol, Professional Information Brochure, Zeneca Pharmaceuticals, April 2001). Propofol was originally marketed as the emulsion with no preservative. However, concerns over potential contamination problems observed after approval of the NDA led the manufacturer to withdraw the formulation and replace it with one containing the preservative EDTA.
A generic formulation has recently been approved and marketed, in which the pH is lowered to the range of 4.5 to 6.4, and the preservative EDTA has been replaced with sodium metabisulfite at 0.25 mg/mL. In the absence of a solubilizing agent, the water solubility of propofol is approximately 0.154 mg/mL. Thus these two formulations are both prepared as oil in water emulsions, with most of the propofol being solubilized by the lipid phase. The preservatives have been added to inhibit microbial growth. The EDTA formulations are disclosed in U.S. Pat. No. 5,714,520, No. 5,731,355, No. 5,731,356 and No. 5,908,869 to Jones et al.
A number of other patents disclose different propofol formulations reportedly having improved stability as compared to conventional emulsion-type formulations. U.S. Pat. No. 6,028,108 to George discloses an oil-in-water emulsion formulation containing propofol and pentetate. U.S. Pat. No. 6,100,302 to Pejaver et al. discloses an oil-in-water formulation containing propofol and soybean oil. U.S. Pat. Nos. 6,140,373 and 6,140,374 to May et al. discloses an oil-in-water formulation containing propofol and an antimicrobial agent. U.S. Pat. No. 6,147,122 to Mirejovsky et al. discloses an oil-in-water emulsion containing propofol and sodium bisulfite, potassium metabisulfite, potassium sulfite, or sodium sulfite as antioxidant or antimicrobial preservative. U.S. Pat. No. 5,637,625 to Haynes discloses a microdroplet formulation containing propofol. None of these references suggests or discloses the non-emulsion formulation of the present invention.
Concerns also exist regarding the potential for allergies to the egg components of the emulsion and to the bisulfite used as a preservative in the generic formulation, as well as the potential for hyperlipidemia such as the propofol infusion syndrome reported in children (Bray, R. J., “Propofol infusion syndrome in children” Paediatr. Anaesth. (1998); 8;491–499). Elimination of the lipids and the emulsion formulation would lead to a superior and potentially safer product.
The marketed formulation of propofol is associated with significant incidence of pain on injection (P. Picard et al. in Anesth. Analg. (2000), vol. 90, pp. 963–969). The incidence of painful reactions after injection of propofol into the small dorsal veins of the hand is 30%–70% (R. A. Johnson et al. in Anaesth. (1990), 45:439–442; P. Barker et al. in Anaesth. (1991) 46:1069–1070; S. Y. King et al. in Anesth. Analg. (1992) 74:246–249). With injection into larger proximal veins, the probability of a painful reaction is 0%–30% (M. J. McCullought et al. in Anaesthesia (1985) 40:1117–1120; R. P. Scott et al. in Anaesthesia (1988) 43:92–94; C. H. McLeskey et al. in Anesth. Analg. (1993) 77(Suppl):S3–9).
Numerous approaches have been tested in an attempt to reduce the incidence of side effects associated with administration of propofol by injection. Some of these methods include: 1) dilution of the propofol formulation with 5% glucose and slow administration of the resulting mixture (D. N. Stokes et al., “Effect of diluting propofol on the incidence of pain on injection”, Anaesth. (1989) 62:202–203); 2) use of cold isotonic saline (P. Barker et al., “Effect of prior administration of cold saline on pain during propofol injection: A comparison with cold propofol and propofol with lignocaine”, Anaesth. (1991) 46:1069–1070); 3) pretreatment of the injection site with another agent (S. Y. King et al., “Lidocaine for the Prevention of Pain Due to Injection of Propofol”, Anesth. Analg. (1992) 74:246–249; M. E. Nicol et al., “Modification of Pain on Injection of Propofol—A Comparison between Lignocaine and Procaine”, Anaesthesia (1991) 46:67–69; W. A. Alyafi et al., “Reduction of Propofol Pain—Fentanyl vs Lidocaine”, Middle East J. Anesthesiol. (1996) 13:613–619; N. M. Gajraj et al., “Preventing Pain During Injection of Propofol: The Optimal dose of Lidocaine”, J. Clin. Anesth. (1996) 8:575–577; D. S. McDonald et al., “Injection Pain with Propofol. Reduction with Aspiration of Blood”, Anaesthesia (1996) 51:878–880; M. H. Nathanson et al., “Prevention of Pain on Injection of Propofol: A Comparison of Lidocaine with Alfentanil”, Anesth. Analg. (1996) 82:469–471; R. D. Haugen et al., “Thiopentone Pretreatment for Propofol Injection Pain in Ambulatory Patients”, Can. J. Anaesth. (1995) 42:1108–1112, 1995; M. Dru et al., “The Effect of Alfentanil on Pain Caused by the Injection of Propofol During Anesthesia Induction in Children”, Can. Anesthesiol. (1991) 39:383–386; R. P. Scott et al., “Propofol: Clinical Strategies of Preventing the Pain on Injection”, Anaesthesia (1988) 43:92–94; D. Wilkinson et al., “Pain on Injection of Propofol: Modification by Nitroglycerin”, Anesth. Analg. (1993) 77:1139–1142); 4) cooling of the propofol containing solution to 4° C. prior to injection (A McCrirrick et al., “Pain on Injection of Propofol: The Effect of Injectate Temperature”, Anaesthesia (1990) 45:443–444); 5) warming of the propofol containing solution to 37° C. prior to injection (G. C. Fletcher et al., “The Effect of Temperature Upon Pain During Injection of Propofol”, Anaesthesia (1996) 51:498–499); 6) mixing of the propofol containing solution with another active agent (G. Gehan et al., “Optimal Dose of Lignocaine for Preventing Pain on Injection of Propofol”, Br. J. Anaesthes. (1991) 66:324–326; G. Zaouk et al., “Alizaprode Does Reduce Pain on Injection of Propofol—Comparison with Lidocaine”, Br. J. Anaesth. (1993) 70:P6; B. Lyons et al., “Modification of Pain on Injection of Propofol. A Comparison of Pethidine and Lignocaine”, Anaesthesia (1996) 51:394–395); and 7) adjustment of the pH of the propofol containing solution prior to injection (M. Eriksson et al., “Effect of lignocaine and pH on propofol-induced pain”, Br. J. Anaesth. (1997) 78:502–506).
A number of researchers have investigated the hypothesis that the pain is associated with the concentration of free (not located in the lipid phase) propofol in the formulation. Doenicke, et. al. (Reducing Pain During Propofol Injection; The Role of the Solvent Anesth. Analg. (1996) 82:472–474) added saline or increasing amounts of long chain triglyceride emulsion to the marketed formulation in an attempt to shift the free propofol into the added lipid. The formulations were administered into a dorsal vein of the hand and the patients reported their pain during injection as none, mild, moderate or severe. The results indicate that pain was decreased as the lipid content in the formulation increased (and the concentration of propofol in the aqueous phase decreased).
Although several of these approaches have shown the ability to reduce the pain on injection observed with propofol, all have the disadvantage of requiring additional manipulation, which may or may not alter the pharmacokinetics and pharmacodynamics and makes delivery of anesthetics less efficient. Furthermore, the use of additional excipients may cause other side effects, such as hyperlipidemia. From a pharmacoeconomic viewpoint, additional manipulation and/or use of added components only increases the cost of treatment. Clearly a single, simple formulation demonstrating reduced pain on injection would be preferred.
Any reformulation of a drug has the ability to alter the drug's pharmacokinetics and pharmacodynamics. The potential for this is an obvious concern when one replaces an emulsion formulation with a true solution formulation. Several investigators have evaluated the effects of different emulsion or Cremophor® formulations on the pharmacokinetics (PK) and pharmacodynamics (PD) of propofol in rats (S. Dutta et al. in J. Pharm. Sci. (1997) 86(8):967–968; E. H. Cox et al. in Pharm. Res. (1998) 15(3):442–448). In a recent study (T. W. Schnider et al. in Anesth. (1998) 88:1170–1182), the effects of the additive EDTA, age, and method of administration on PK/PD were also examined in humans. The conclusion reached in each of these studies was that there were no changes in the PK/PD between the different formulations. There were, however, changes as a result of the method of administration (bolus vs. infusion).
U.S. Pat. No. 4,452,817 to Glen et al. discloses parenteral formulations containing propofol and solid diluent, a sterile water miscible solvent, an aqueous solution and/or a surfactant.
When the comparison was made between the marketed formulation and an in-situ prepared “lipid-free” formulation, there was a tendency for the lipid free formulation to demonstrate delayed onset of action and recovery in rats (S. Dutta et al. in Anesth. (1997) 87(6):1394–1405). This was confirmed in additional studies where an increase in volume of distribution and reduced potency was also seen with the lipid free formulation (S. Dutta et al. in J. Pharm. Pharmacol. (1998) 50:37–42). In each of these last two studies, the lipid free formulation was formed at the point of entry into the body. The propofol was diluted in ethanol and pumped into a mixing tee tube. A carrier solvent of water, glycerol and dextrose was pumped into the tee from another pump. The mixture passed from the tee directly to the rat. There was no confirmation that a true solution existed. It is very possible that propofol did not remain in solution, and that fact alone could explain the observed changes in PK/PD.
Numerous studies have reported on the success of formulations containing cyclodextrins and cyclodextrin derivatives to reduce tissue damage and pain following intramuscular injection (T. Irie et al. J. Pharmacobio-Dyn. (1983) 6(10):790–2; K. Masuda et al. in Yakugaku Zasshi (1984) 104(10):1075–82; A. Yoshida et al. in Chem. Pharm. Bull. (1990) 38(l):176–9) and intradermal injection (U.S. Pat. No. 5,602,112 to J. Rubinfeld). The intramuscular studies evaluated the tissue irritation after drugs were administered as suspensions in saline, or solubilized as complexes with β-cyclodextrin or 2-hydroxypropyl-β-cyclodextrin in water. The formulations containing the drugs complexed with the cyclodextrins showed reduced visual signs of irritation and tissue damage as compared to the formulations in saline. No assessment was made of pain. These studies show reduced tissue damage from cyclodextrin complexation, but only after 2 days of localized contact. The intradermal studies evaluated the ulcerative effects of several cytotoxic compounds, formulated with or without cyclodextrins, after administration into the skin of rats. Again, no measurement of pain was taken and the irritation evaluation was conducted only after contact times of 1 to 20 days. None of these studies evaluated the effects of cyclodextrin complexation on the pain associated with injection, especially after rapid intravenous administration or a continuous intravenous infusion.
Cyclodextrins are cyclic carbohydrates derived from starch. The unmodified cyclodextrins differ by the number of glucopyranose units joined together in the cylindrical structure. The parent cyclodextrins contain 6, 7, or 8 glucopyranose units and are referred to as α-, β-, and γ-cyclodextrin respectively. Each cyclodextrin subunit has secondary hydroxyl groups at the 2 and 3 positions and a primary hydroxyl group at the 6 position. The cyclodextrins may be pictured as hollow truncated cones with hydrophilic exterior surfaces and hydrophobic interior cavities. In aqueous solutions, these hydrophobic cavities provide a haven for hydrophobic organic compounds that can fit all or part of their structure into these cavities. This process, known as inclusion complexation, may result in increased apparent aqueous solubility and stability for the complexed drug. The complex is stabilized by hydrophobic interactions and does not involve the formation of any covalent bonds.
This dynamic and reversible equilibrium process can be described by Equations 1 and 2, where the amount in the complexed form is a function of the concentrations of the drug and cyclodextrin, and the equilibrium or binding constant, Kb. When cyclodextrin formulations are administered by injection into the blood stream, the complex rapidly dissociates due to the effects of dilution and non-specific binding of the drug to blood and tissue components.
                    Drug        +                  Cyclodextrin          ⁢                      ⟷                          K              b                                ⁢          Complex                                    Equation        ⁢                                  ⁢        1                                          K          b                =                              [            Complex            ]                                              [              Drug              ]                        ⁡                          [              Cyclodextrin              ]                                                          Equation        ⁢                                  ⁢        2            
The underivatized parent cyclodextrins are known to interact with and extract cholesterol and other membrane components, particularly upon accumulation in the kidney tubule cells, leading to toxic and sometimes fatal renal effects. Chemical modification of the parent cyclodextrins (usually at the hydroxyls) has resulted in derivatives with improved safety while retaining or improving the complexation ability. Of the numerous derivatized cyclodextrins prepared to date, only two appear to be commercially viable: the 2-hydroxypropyl derivatives (HP-β-CD; neutral cyclodextrins being commercially developed by Janssen and others), and the sulfoalkyl ether derivatives, such as sulfobutyl ether, (SBE-CD; anionic cyclodextrins being developed by CyDex, Inc.) However, the HP-β-CD still possesses toxicity that the SBE-CD does not.
The PK/PD of cyclodextrin containing propofol formulations has been evaluated. In a comparison study of the marketed formulation with one containing HP-β-CD, the effects of the formulation on cardiac changes in the rat were monitored. A substantial bradycardia of short (1–6 seconds) duration was observed following administration of the HP-β-CD formulation (S. J. Bielen et al. in Anesth. Analg. (Baltimore) (1996) 82(5):920–924). Bradycardia was not observed with the marketed formulation. A comparable study in rabbits showed no changes in PK or PD for the HP-β-CD containing formulation (H. Viernstein et al. in Arzneim.-Forsch. (1993) 43(8):818–21). However, there have been many reports of sudden bradycardia and asystole in patients under propofol anesthesia using the marketed formulation (T. D. Egan et al. in Anesth. Analg. (1991) 73:818–820; M. F. M. James et al. in Br. J. Anaesth. (1989) 62:213–215). Thus the observed bradycardia is unpredictable and the success or failure of one cyclodextrin-propofol combination does not predict the results that would be obtained with the success or failure of another cyclodextrin-propofol combination.
In a more recent study, the complexation of propofol with HP-β-CD was studied by physicochemical methods, nuclear magnetic resonance, spectroscopic methods, and by a comparison of the anesthetic properties in rats of the uncomplexed propofol (Diprivan® emulsion-type formulation) to that of the complexed propofol (G. Trapani et al. in J. Pharm. Sci. (1998), 87(4):514–518). Trapani et al. determined there are significant differences between the two formulations in the induction time and sleeping time. In the field of generic drugs where bioequivalence between a currently approved product and a generic product is necessary, the HP-β-CD/propofol formulation could not be approved due to the lack of bioequivalence. They suggested the possibility that the rats did not feel pain on injection but provided no data to support the suggestion.
International Publication No. WO 96/32135 to FARMARC et al. discloses parenteral and enema formulations comprising propofol and HP-β-CD. FARMARC et al. state that the preferred molar ratio of propofol to HP-CD is about 1:2 to about 1:2.5 (which corresponds to a 1:15.75 to 1:19.65 wt./wt. ratio) in the absence of a cosolvent in order to obtain a clear colorless solution. The ratio can be reduced to 1.5 to <2.0 (which corresponds to a 1:11.79 to 1:15.75 wt./wt. ratio) if a cosolvent (such as glycol, propylene glycol, or polyethylene glycol) is added. FARMARC et al. also state that propofol concentrations of only up to 30 mg/mL can be achieved; however, they recognize that the toxic properties of HP-β-CD limits the practical concentration of HP-CD to 215 mg/mL. Accordingly, the art recognizes the undesired toxicity of HP-β-CD based formulations of propofol.
A sulfobutyl ether derivative (SBE-β-CD), in particular the derivative with an average of about 7 substituents per cyclodextrin molecule, is being commercialized by CyDex, Inc. as CAPTISOL®. The anionic sulfobutyl ether substituent dramatically improves the aqueous solubility of the parent cyclodextrin. In addition, the presence of the charges decreases the ability of the molecule to complex with cholesterol as compared to the hydroxypropyl derivative. Reversible, non-covalent, complexation of

Sulfobutyl Ether-β-Cyclodextrin (Captisol®) drugs with CAPTISOL® cyclodextrin generally allows for increased solubility and stability of drugs in aqueous solutions. While CAPTISOL® cyclodextrin is a relatively new but known cyclodextrin, its combined use with propofol in parenteral formulations and its effect upon the behavior of propofol when administered parenterally has not previously been evaluated.
The safety of cyclodextrins is often compared by way of in-vitro hemolysis studies. As depicted in FIG. 1 (Thompson, D. O., Critical Reviews in Therapeutic Drug Carrier Systems, (1997), 14(1), 1–104), the hemolytic behavior of the CAPTISOL® cyclodextrin is compared to the same for the parent β-cyclodextrin, the commercially available hydroxypropyl derivatives, ENCAPSIN™ cyclodextrin (degree of substitution3–4) and MOLECUSOL® cyclodextrin (degree of substitution˜7–8), and two other sulfobutyl ether derivatives, SBE1-β-CD and SBE4-β-CD. Unlike the other cyclodextrin derivatives, SAE-CD derivatives, in particular those such as the CAPTISOL® cyclodextrin (degree of substitution˜7) and SBE4-β-CD (degree of substitution˜4), show essentially no hemolytic behavior in concentrations typically used to solubilize pharmaceutical formulations. These SAE-CDs exhibit substantially lower membrane damaging potential than the commercially available hydroxypropyl derivatives.
Sulfated cyclodextrin derivatives have also been prepared and their effects on blood lotting time evaluated. Sulfated cyclodextrins were found to interfere significantly with blood clotting time, especially when compared to the sulfoalkyl ether cyclodextrins (Thompson, D. O., Critical Reviews in Therapeutic Drug Carrier Systems, (1997), 14(1), 1–104).
Methylated cyclodextrins have been prepared and their hemolytic effect on human erythrocytes has been evaluated. These cyclodextrins were found to cause moderate to severe hemolysis (Jodal et al., Proc. 4th Int. Symp. Cyclodextrins, (1988), 421–425; Yoshida et al., Int. J Pharm., (1988), 46(3), 217–222).
The osmolality of a formulation is generally associated with its hemolytic potential: the higher the osmolality (or the more hypertonic), the greater the hemolytic potential. Zannou et al. (“Osmotic properties of sulfobutyl ether and hydroxypropyl cyclodextrins”, Pharma Res. (2001), 18(8),) compared the osmolality of solutions containing SBE-CD and HP-CD. As depicted in FIG. 2, the SBE-CD containing solutions have a greater osmolality than HP-CD containing solutions comprising similar concentrations of cyclodextrin derivative.
Of the different cyclodextrins mentioned above, the sulfoalkyl ether cyclodextrins are most suitable for parenteral administration.
Thus, in the field of sedative hypnotic therapy, there remains the need for improved injectable formulations that have a reduced or eliminated incidence of pain upon injection, enhanced stability, minimal potential for allergic reaction and microbial growth, and/or minimal cardiac side effects caused by the formulation.